In general, when a lubricating base oil is produced from crude oil, the crude oil is first subjected to atmospheric distillation, and the resulting residual oil is further subjected to vacuum distillation to separate various lubricating oil fractions having varied viscosities and vacuum distillation residual oil. The vacuum distillation residual oil is subjected to solvent deasphalting, thereby removing asphalt contents and obtaining a heavy lubricating oil fraction (bright stock). These lubricating oil fractions having varied viscosities, including the bright stock, are further subjected to solvent refining, hydrofinishing, dewaxing and the like steps to produce the lubricating base oil of interest.
On the other hand, a hydrocracking process is known as a process for the production of a lubricating base oil having a high viscosity index. In this process, a vacuum gas oil fraction, a bright stock, wax of various types or a mixture thereof is subjected to hydrocracking under high temperature and high pressure conditions in the presence of a catalyst, and a high viscosity index base oil is produced from the resulting oil.
Examples of the hydrocracking of heavy oil are disclosed, for instance, in JP-B-46-3267, JP-B-50-26561, JP-B-50-36442, JP-B-51-15046, JP-B-51-41641, JP-B-54-21205, JP-B-54-31002, JP-B-57-17912, JP-B-62-5958, JP-A-48-49804, JP-A-63-258984, JP-A-64-6094, JP-A-3-197594, JP-A-3-223393 and the like. (The term "JP-B" as used herein means an "examined Japanese patent publication", and the term "JP-A" as used herein means an "unexamined published Japanese patent application".) Also, hydrocracking and isomerization of wax and the like as the stock oil are disclosed, for instance, in JP-B-57-17037, JP-B-60-22039, JP-A-50-92905, JP-A-51-146502, JP-A-52-136203, JP-A-1-223196, JP-A-1-301790, JP-B-4-503371, JP-A-4-226594, U.S. Pat. No. 4,547,283, U.S. Pat. No. 4,906,350, EP-A1-0464547 and the like.
Development of a low viscosity base oil having a high viscosity index has been called for in the area of not only engine oil but also hydraulic fluid for construction machine use.
However, production of a low viscosity lubricating base oil having a high viscosity index is not easy because, when it is produced by the solvent refining process in the art, the product is limited to certain lubricating oil fractions from specific high quality crude oil, and an extremely high extractant ratio is required in the solvent refining step.
Also, since heavy oils such as vacuum gas oil fractions, bright stocks and the like, various types of wax or mixtures thereof are used as the stock oil in the hydrocracking process in the art, the viscosity index of the lubricating oil fractions produced by this process is high in the case of a distillate having a relatively high viscosity, but the index is not so high when the fraction has a relatively low viscosity of 3.0 to 7.5 mm.sup.2 /s as a kinematic viscosity at 100.degree. C.
In consequence, the hydrocracking process in the art aims at producing a lubricating base oil having a relatively high viscosity and, therefore, is not suitable for the production of a lubricating base oil having a relatively low viscosity and a high viscosity index.
In the case of the catalytic isomerization of slack wax, on the other hand, it is necessary to carry out a pretreatment for the removal of nitrogen and sulfur components by arranging a hydrofining step prior to the isomerization step, because the isomerization catalyst is apt to cause deterioration due to nitrogen and sulfur compounds contained in the slack wax.